This application is related to Japanese application No. HEI 11 (1999)-224065 filed on Aug. 6, 1999, whose priority is claimed under 35 USC xc2xa7 119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a coating liquid for forming an organic layer in an organic LED display and a method of manufacturing an organic LED display. More particularly, it relates to the manufacture of an organic LED display by an ink-jet technique, a coating liquid capable of preventing a nozzle from clogging and providing an organic LED display in a successive and stable manner, and a method of manufacturing an organic LED display using the coating liquid.
2. Description of Related Art
An organic LED display which includes a light-emitting layer of a polymer material has been manufactured by spin coating. However, since this technique cannot pattern the light-emitting layer, it has not been reported yet that the technique is applied for the formation of a full-color display panel.
Recently, however, as means of patterning the light-emitting layer, methods of manufacturing a light-emitting layer which produces the three primary colors by an ink-jet technique have been proposed (Japanese Unexamined Patent Publications Nos. HEI 10(1998)-12377, HEI 10(1998)-153967, HEI 11(1999)-54270 and HEI 11(1999)-54272, Appl. Phys. Lett. 72, 519, 1998, and Appl. Phys. Lett. 72, 2660, 1998).
Conventionally, highly volatile solvents such as dichloromethane, dichloroethane and methanol have mostly been used as solvents for dissolving the polymer material, though sometimes water has been used. Accordingly, when pixels are formed by the ink-jet technique using such solvents, the polymer material is dried very quickly after it is discharged. As a result, the resulting layer is not formed flat, which makes the light-emission of the organic LED display irregular.
Further, where the light-emitting layer is formed by discharging the coating liquid in plural times, the resulting layer will form interfaces therein because of the same reason described above. Thus, electrical characteristics are deteriorated.
In a field of conventional ink-jet printing, a solvent of low vapor pressure is used as measures for dealing with the above drawback. It is known that such a solvent of low vapor pressure has a high boiling point. In the usual printing, the solvent is absorbed into paper so that the existence of the remaining solvent is less significant. However, in the manufacture of the organic LED display, a coating liquid is discharged on a glass substrate or the like and the remaining solvent may cause deterioration of device characteristics. Therefore, the remaining solvent must be completely removed in the manufacture of the organic LED display. Thus, the solvent needs to be evaporated by heat drying (Japanese Unexamined Patent Publication No. HEI 11(1999)-54272). However, the publication employs a drying temperature of not higher than 120xc2x0 C., at which the low volatile solvent cannot be completely removed.
When the solvent is removed by heat drying at a high temperature, the polymer material is also heated. As a result, the polymer material is decomposed or denatured, which leads to a destruction or deterioration of the device. Further, when a multi-layered film of polymer materials is formed, the material of the layer to be heated and that of a layer contacting it are mixed at an interface therebetween, which causes the deterioration of the device characteristics.
The formation of the organic LED display by the ink-jet technique has also suffered from clogging of a nozzle. To solve this clogging problem, Yoshimori et al. have proposed a method of mixing isopropylalcohol in the solvent (the 59th Science Lecture of Appl. Phys. Soc., 16p-YH-15). However, the clogging has not been completely prevented by this method.
The inventors of the present invention have deeply studied the above problems, and as a result, found that an organic layer with a flat surface and free from forming interfaces therein can be formed while preventing the clogging of the nozzle by using the ink-jet technique and a coating liquid containing an organic solvent of particular characteristics for forming an organic layer of an organic LED display. Further, it has also been found that the remaining solvent can be completely removed without the destruction and deterioration of the device by subjecting a film formed by using the coating liquid to a predetermined heating step, and thus the irregular light-emission can be dissolved.
According to the present invention, provided is a coating liquid for forming an organic layer of an organic LED display which layer includes a light-emitting layer or a light-emitting layer and a charge-transporting layer, the liquid comprising: an organic material or a precursor thereof required for forming the organic layer, and at least one low volatile liquid solvent having a vapor pressure of 10 mmHg or less at 20xc2x0 C. and a boiling point lower than a denaturing temperature of the organic material or lower than a converting temperature of the precursor.
Further, according to the present invention, provided is a method of manufacturing an organic LED display comprising the steps of: discharging the above-mentioned coating liquid for forming the organic layer of the organic LED display by an ink-jet technique to form at least one film, and heat drying the film at a temperature not lower than the boiling point of a low volatile liquid solvent contained in the coating liquid, thereby forming the organic layer.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.